Recent advances in the liquid crystal panel manufacturing techniques using thin film transistors (TFTs), and utilization of an area server having a semiconductor conversion element in various fields (e.g., in the field of a medical X-ray image pick-up device), have enabled increased surface area and digitization for medical radiation image pick-up devices as well. The medical radiation image pick-up device, unlike a liquid crystal panel and the like, has a feature in that a minute signal is digitized to output a corresponding image, and hence can photograph an image of radiation instantaneously to display the photographed image on a display device in an instant. At the present time, as for such a radiation image pick-up device, one for still image photographing has been commercialized.
FIG. 11 is a schematic plan view schematically showing an example of a conventional radiation image pick-up device, FIG. 12 is an equivalent circuit diagram of the conventional radiation image pick-up device shown in FIG. 11, and FIG. 13 is an equivalent circuit diagram of one pixel and a signal reading circuit in the conventional radiation image pick-up device shown in FIG. 11 (refer to JP 8-116044 A for example). Hereinafter, a case where an image of X-ray as radiation is photographed will be described.
As shown in FIG. 11, the conventional radiation image pick-up device includes a sensor substrate 101 in which a plurality of pixels each having a photoelectric conversion function are disposed, a scanning circuit 102 for scanning the pixels, a signal output circuit 103 for outputting signals from the pixels, ICs 104 through which the sensor substrate 101 and the scanning circuit 102 are connected to each other, and ICs 105 through which the sensor substrate 101 and the signal output circuit 103 are connected to each other.
As shown in FIG. 12, a plurality of pixels 106 are disposed in matrix in the sensor substrate 101. Note that 3×4 pixels are illustrated in the pixel area for the sake of convenience in FIG. 12, in actuality, a large number of pixels, 1,000×2,000 pixels for example, are disposed therein. In addition, similarly, the illustration of ICs of the scanning circuit is omitted here for the sake of convenience.
As shown in FIGS. 12 and 13, each pixel 106 is constituted by a photoelectric conversion element 111 as a semiconductor element for converting incident X-rays into electric charges, and a thin film transistor (TFT) 112 acting as a switching element for reading out the resultant electric charges.
In each pixel 106, the photoelectric conversion element 111 is connected to the signal output circuit 103 through a bias line 110 which is common to all the pixels, and thus a constant bias voltage is applied from the signal output circuit 103 to the photoelectric conversion element 111. In addition, in each pixel 106, a gate electrode of the TFT 112 is connected to the scanning circuit 102 through the IC 104 (not shown) and a gate line 113 which is common to every row in the matrix. Thus, the scanning circuit 102 controls an operation (turn-ON/turn-OFF) of the TFT 112. In addition, in each pixel 106, a source or drain electrode of the TFT 112 is connected to the signal output circuit 103 through the IC 105 by way of a signal reading wiring (signal line) 114 which is common to every column in the matrix.
As shown in FIGS. 12 and 13, the IC 105 includes an amplifier 115 serving as a signal reading circuit. One input terminal of the amplifier 115 is connected to the signal line 114, and the other input terminal thereof is connected to a power supply 116. Moreover, a gain switching circuit 117 having capacitors Cf1, Cf2, and Cf3 is connected to the amplifier 115, and thus a gain of the amplifier 115 can be switched over to another one through the combination of the capacitors Cf1, Cf2, and Cf3.
Here, as shown in FIG. 13, a capacity of the photoelectric conversion element 111 is assigned C1, a parasitic capacity of the signal line 114 is assigned C2, and a capacity of the amplifier 115 is assigned Cf. An X-ray applied to a subject for exposure is attenuated as it is transmitted through the subject to be wavelength-converted into visible light by a phosphor layer (wavelength conversion member)(not shown). The resultant visible light is then made incident on the photoelectric conversion element 111 to be converted into electric charges Q.
Subsequently, upon turn-ON of the TFT 112, the gain of 1/Cf-fold is set in the amplifier 115. As a result, an output voltage Vout is expressed by Vout=−Q/Cf, and this voltage signal is then read out from the signal output circuit 103 to the outside. After completion of the operation for reading out the voltage signal Vout, the electric charges which are generated in the photoelectric conversion element 111 but remained untransferred are removed due to a change in electric potential of the common bias line 110.
However, the above-described conventional radiation image pick-up device principally aims at photographing a still image, and hence sensitivity (S/N ratio) is fixed at constant. Thus, the S/N ratio may become insufficient depending on the photographing modes. That is, the conventional radiation image pick-up device involves a problem in that it has little tolerance for differences in the attenuation of the X-rays between different subjects, or for large differences in the dosage of exposure to the X-rays such as when still image photographing and moving image photographing are performed.